1. Field of the Invention
This invention relates to a variable AC-to-DC converter, and a method for operating a variable-DC power supply, particularly to AC-to-DC converters employing pulse-width modulation (PWM) conversion methods.
2. Description of the Art
In general, existing AC/DC power conversion is accomplished using a variety of circuit topologies with phase-controlled SCRs operating from multi-phase transformers. These topologies may sometimes have the drawbacks of harmonic line currents, a variable power factor, and high DC ripple currents. These traits are especially problematic at higher power levels typical for arc loads and plasma torches. Because of these and other disadvantages of the SCR circuit topology, pulse-width modulated (PWM) circuits are preferred to provide smoother control.
Pulse-width modulation is a form of modulation in which the value of each instantaneous sample of the modulating wave is caused to modulate the duration of a pulse. In PWM, the modulating wave may vary the time of occurrence of the leading edge, the trailing edge, or both edges of the pulse. The modulating frequency may be fixed or variable. In a PWM circuit, a reference signal may be used to generate a train of pulses, the width of each pulse being related to the instantaneous value of the referenced signal. The pulses may be generated by using a comparator to compare the reference signal with a carrier signal, which may be a sawtooth or triangular wave. When the reference signal exceeds the carrier signal, the output of the comparator is high; at other times, the output of the comparator is low. The comparator output does provide a train of pulses representing the reference signal. The pulses are then used to drive an electronic switching device for intermittently applying a voltage across the load.
When a voltage is suddenly applied across an inductive and resistive load, such as a plasma torch, the current through the load rises almost linearly with time. When the voltage is then turned off, the current through the load does not immediately fall to zero but decreases approximately linearly with time, as the inductor's magnetic field collapses, and the current flows in a freewheeling diode. Thus, the input voltage pulses applied across the load result in a current which has a ripple. This ripple is inherent in all PWM amplifiers. The magnitude of the ripple is directly proportional to the supply voltage and inversely proportional to the switching frequency and to the circuit inductance. Current ripple is generally undesirable because it wastes energy in the inductor and may cause unwanted pulsations in the load. To reduce ripple, the SCR controller typically increases the inductance in series with the load as well as increase chopping to a 12 or 24 pulse output. This is typically an expensive approach and further increases losses and injects harmonics via line notching on the power system. Pulse-width modulation techniques can reduce ripple by interdigitating multiple power cells to significantly increase the ripple frequency and reduce the ripple amplitude.